Cellular communication systems are known. Such systems, typically, provide communication services throughout a broad geographic area through a number of base sites distributed throughout the geographic area. The base sites of the system, and associated coverage areas, are arranged within the geographic area in such a manner to provide substantially seamless coverage for communication units passing through the geographic area by being passed from one base site to another.
Base sites within the cellular system offer communication services to communication units within their service coverage areas on a number of communications frequencies (communication resources) (f.sub.1 -f.sub.n) reserved for such use. Such communication services are facilitated by the transmission of an identification signal by a base site. The identification signal is used by communication units as a means of identifying proximate base sites through which service may be requested.
Service within the communication system is provided to requesting communication units through a proximate base site upon receipt of an access request and upon verification that the communication unit is registered within the system. Notification of service may be provided to the communication unit by the base site transmitting a resource allocation to the communication unit. The base site may also allocate a transceiver and voice path through the base site providing access to a communication target.
The communication resource allocated for use by the communication unit is typically in the nature of a duplex traffic channel that, in the case of a frequency division multiple access system, may be a voice channel comprised of a frequency pair (transmit and receive). In other systems the duplex voice channel may exist within a common spectrum using spreading codes (as with a code division multiple access (CDMA) communication system). In still other systems the duplex voice channel may be comprised of slots on a transmit and receive frequency of a time division multiple access (TDMA/TDM) system such as, for example, the Global System for Mobile Communications (GSM) Pan-European digital cellular system, as specified in GSM recommendations available from the European Telecommunications Standards Institute (ETSI). Under GSM a number of voice channels may exist within the slots of a repeating GSM frame.
As a means of reducing in reference within a GSM-like system the frequency of the traffic channel, over which the duplex voice signal is being exchanged with the base site, may be changed after each exchanged frame. Operating frequencies for each frame (over successive hopped frames), in such case, may be generated by a hopping algorithm such as, for example, GSM recommendation 6.2.3. The use of frequency hopping within a GSM system brings such a system within the definition of CDMA.
As the duplex voice signal is maintained, signal measurements are used to detect movement of the communication unit relative to the base site. As the communication unit moves out of the coverage area of a serving base site such movement must be detected and another base site selected for handoff before signal loss occurs.
The need for handoff in some systems may be detected by measurement of a signal quality factor (such a received signal strength indication (RSSI)) of the served communication unit by base sites surrounding the served communication unit. In other systems the served communication unit measures signals of surrounding base sites, using a method referred to as mobile assisted handoff (MAHO). In MAHO systems the most reliable signal for signal quality measurements is the control channel used by the measured base site.
MAHO signal measurements are transferred to the serving base site by the communication unit on a slow associated control channel (SACCH) used in conjunction with a traffic channel. Such signal measurements may be, in turn, transferred to a mobile switching center (MSC) for a determination of a handoff target. Handoff, in such case, is accomplished by the serving base site transmitting a handoff command to the served communication unit.
While handoff, under MAHO, has worked well in the past, such success has been based upon readily available control signals of surrounding base sites located on fixed frequencies, identifiable through a minimum of scanning or information transfer from a serving base site. Such availability is necessary because of the limited time that a communication unit has between communicated frames for scanning for nearby base sites.
In MAHO systems using frequency hopping, such hopping has been confined to traffic channels, with control channels existing on fixed frequencies. Because of the importance of frequency hopping in reducing interference, a need exists for a method of using MAHO in connection with frequency hopping control channels.